A MODEL FOR EXPLAINING THE MÖSSBAUER SPECTRA OF FERRIMAGNETIC SPINELS WITH A HIGH DEGREE OF DIAMAGNETIC SUBSTITUTION

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A MODEL FOR EXPLAINING THE MÖSSBAUER SPECTRA OF FERRIMAGNETIC SPINELS WITH A HIGH DEGREE OF DIAMAGNETIC SUBSTITUTION

E. de Grave, A. Govaert, D. Chambaere, G. Robbrecht

To cite this version:

E. de Grave, A. Govaert, D. Chambaere, G. Robbrecht. A MODEL FOR EXPLAINING THE MÖSSBAUER SPECTRA OF FERRIMAGNETIC SPINELS WITH A HIGH DEGREE OF DIA- MAGNETIC SUBSTITUTION. Journal de Physique Colloques, 1979, 40 (C2), pp.C2-669-C2-670.

�10.1051/jphyscol:19792232�. �jpa-00218614�

JOURNAL DE PHYSIQUE Colloque C2, suppl6ment au n

"

3, Tome 40, mars 1979, page C2-669

A MODEL FOR E X P L A I N I N G THE MOSSBAUER SPECTRA OF FERRIMAGNETIC S P I N E L S WITH A H I G H DEGREE OF DIAMAGNETIC SUBSTITUTION

E. De Grave, A. Govaert, D. Chambaere and G. Robbrecht

Laboratory of Magnetism, lhziversity

OF

^{Ghent, B} 9000 Ghent, Belgium

Rdsum6.- Nous avons Qtudid par spectromQtrie Mgssbauer du 5 7 ~ e les solutions solides Fe2fl-y)Mgl+y Ti 0, avec y 2 0.4 de structure spinelle. Les propriQt6s magndtiques sont dominQes par 1 existence d'amas de moment magndtique gCant. Leurs dimensions, dont une estimation a Qtd obtenue par la thQo- Y rie du superparamagnCtisme, semblent diminuer avec l'augmentation de la concentration en ions non- magndtiques. De plus, la formation des amas est facilitde si l'on rdduit seulement le nombre moyen des paires AB.

Abstract

.-

The magnetic behaviour, as observed with "Fe Mijssbauer spectroscopy, of Fez (l-y)Mgl+y Tiyo, spinels with y 5 0.4 is dominated by giant moment magnetic clusters. Using the theory of su- perparamagnetism, the size of the clusters could be calculated and was found to decrease with in- creasing concentration of non-magnetic ions. It is further shown that cluster formation is enhanced if only the average numbers of AB pairs is decreased.

In a previous paper / I / , we suggested that the simultaneous presence of a strongly broadened six-lines pattern and a central quadrupole doublet, observed in the 5 7 ~ e Mgssbauer spectra of Fe2(l-y) Mgl+yTiyO, spinels with y >, 0.4 may be qualitative- ly explained by the existence of magnetically orde- red regions which are not coupled to each other by inter-sublattice, i.e. AB, superexchange interac- tions. Statistical fluctuations in the distribution of magnetic and non-magnetic ions are believed to be responsible for this behaviour, which was obser- ved in other diluted spinel systems too / 2 , 3 / . The quadrupole doublet then arises from those Fe3+ ions which are situated within regions which are so small that they behave superparamagnetically. From this point of view, it is to be expected that the "ave- rage cluster size" decreases, not only if the ove- rall concentration of magnetic ions decreases but also if the average number of AB superexchange in- teractions decreases without altering the overall magnetic concentration at all.

The composition Fel.4Mgl.3Ti0.304 does not show a superimposed quadrupole doublet, at any tem- perature / I / . By substituting part of the M~'+ for zn2+, less ~ e will be at the tetrahedral sites ~ +

x = 0.5. The influence of the decreasing inter- sublattice interaction on the Miissbauer effect is shown in figure I , from which it is clear that the relative doublet area, increases drastically with increasing x.

-

^{veloc~ty immls)-m}

Fig. 1 : M6ssbauer spectra at room temperature for so that the number n of AB superexchange linkages some Fel.sMg~.s-~Zn~Tio. 304 spinels.

j

per molecule will decrease. Several Fel.4Mgl.3-x This behaviour is however not only due to the lowe- ZnxTio.sOt, samples, with x ranging between 0.0 and rlny of the "average cluster volume" but also partly 0.5 have been prepared by the usual oxide sintering to the lowering of the transition temperature, i.e.

technique. The cation distribution was determined the temperature at which magnetic ordering is no by X-ray diffraction from which it was found that longer observable. Measurements are now in progress

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19792232

c2-670 JOURNAL DE PHYSIQUE these two effects contribute to the observed featu-

res.

It is possible to obtain more information about the magnetic clusters from the temperature dependence of the ratio Np/NTOT of doublet area Np to total area NTOT. For this purpose, the clusters are idealised by single-domain particles with volu- me V and miaxial anisotropy K. The magnetization vector of such a particle rapidly flips between the two directions of easy magnetization and the rela- xation time T of this process increases with in- creasing volume and/or with decreasing temperature /4,5/. The observed quadrupole doublet then arises from those clusters having T smaller than the nu- clear Larmor precession time rL (about 2.5 x IO-'s for "Fe) i.e. from those clusters with a volume smaller than the so-called critical volume Vc, the value of which may be calculated as a function of temperature using the theory of superparamagnetism /4/. The relative doublet intensity may thus be written as :

in which g(V)dV is the volume fraction of the clus- ters with volume between V and V + dV. A simple Gaussian expression for g(V) was found to yield acceptable results.

The experimental and calculated curves N _P / NTOT versus T for some of the investigated spinels are plotted in full line in figure 2 and the quan- titative results are listed in table I. Here, V

g and o are the average volume and spread of the Gaus- sian distribution respectively.

Fig. 2 : Experimental and calculated N /NToT versus T curves for Fel.2Mgi.+Tio.s01, (e), Fel,oMgl.sTio.sO~ P

(4) and Fel.sMg1.1Zno.zTio.30b (A).

It is seen that the average cluster volume shows the expected behaviour when changing the composition of the sample, i.e. that there exists a relation bet- ween V and nJ. This will further be investigated.

g

Finally, it is to note that the observed values of K are typical for superparamagnetic systems.

From the above, preliminary results obtained on the insulating random substitutional Fe

2(1-Y)

Mgl+yTiyO, system, with and without zn2+ substitu- tion, it may be concluded that these materials show a mictomagnetic-like behaviour /6/. The size of the giant moment magnetic clusters could be estimated by relating the theory of superparamagnetism to the ratio of doublet area to total area of the 5 7 ~ e Miissba~ers~ectrum and it is concluded that this size increases with increasing number of Fe

a+ ^-

^{~ e i +}

linkages.

Table I : Quantitative results derived from ~ ~ / N ~ ~ ~ v e r s u s T curves

1 I

Parameter

i

^~e~ .oMgl .sTio.504:Fel.~Mg~.bTio.rOs : ~ e l . ~ ~ g l . I Z ~ O . ~ T ~ O J O I ,

1 I I

I I I

n _J ^I _I 0.24 ^I I 0.36 ^I I 0.46

K(erg/cm3)

i

4.3 X 10'

i

^{4.4 x}

^lo5

^{I I I 6.0 x}

^lo5

v

(cm3)

i

^{4.4 10-l9}

i

^{6.7 10-l9} 1

;

8.9 x 10-1'

g I I

a(cm3) 1 I 2.4 10-l9

i

^{3.4 10-l9}

;

2.9 x 10-1'

I I 1

References

/ 1 / De Grave E., VanLeerberghe,R., Dauwe, C., Govaert, A. and De Sitter, J . J . Physique Colloq.

37

_- (1976) C6-97.

/2/ Basile, F. and Poix, P., Phys. Status Solidi (a)

2

_- (1976) 153.

/3/ Filoti, G., Florescu, V., Nicoara, G. and Spanu, V., Proc. Int. Conf. Mbssb.

Spectr., Bucharest-Romania, Vol.1 (1977) 149.

/4/ Kundig, W., Bonmel, H., Constabaris, G. and Lindquist, R.H., Phys. Rev.

142

(1966) 327.